Problem: Determine how many solutions exist for the system of equations. ${5x-y = -7}$ ${-5x+y = 7}$
Solution: Convert both equations to slope-intercept form: ${5x-y = -7}$ $5x{-5x} - y = -7{-5x}$ $-y = -7-5x$ $y = 7+5x$ ${y = 5x+7}$ ${-5x+y = 7}$ $-5x{+5x} + y = 7{+5x}$ $y = 7+5x$ ${y = 5x+7}$ Just by looking at both equations in slope-intercept form, what can you determine? ${y = 5x+7}$ ${y = 5x+7}$ Both equations have the same slope and the same y-intercept, which means the lines would completely overlap. ${1}$ ${2}$ ${3}$ ${4}$ ${5}$ ${6}$ ${7}$ ${8}$ ${9}$ ${\llap{-}2}$ ${\llap{-}3}$ ${\llap{-}4}$ ${\llap{-}5}$ ${\llap{-}6}$ ${\llap{-}7}$ ${\llap{-}8}$ ${\llap{-}9}$ ${1}$ ${2}$ ${3}$ ${4}$ ${5}$ ${6}$ ${7}$ ${8}$ ${9}$ ${\llap{-}2}$ ${\llap{-}3}$ ${\llap{-}4}$ ${\llap{-}5}$ ${\llap{-}6}$ ${\llap{-}7}$ ${\llap{-}8}$ ${\llap{-}9}$ Since any solution of ${5x-y = -7}$ is also a solution of ${-5x+y = 7}$, there are infinitely many solutions.